Conventional GPR reflection surveys in base metal sulphide mines are sometimes marred by scattering due to heterogeneity or presence of disseminated sulphide minerals. Surface scanning in such areas have its own limitations as the depth of investigation is adversely influenced by the highly lossy medium. Alternatively, borehole GPR investigation from underground can be an efficient tool for sensing the proximity of old workings including cavities left over by past ore extraction. In the present study two borehole GPR techniques were used from underground to establish the boundary of old workings in a Pb-Zn mine. Cross-hole transmission mode data from a stepped-frequency GPR was analysed to delineating the zone of influence created by ancient mining activity. Two parameters namely velocity and attenuation of radar waves were used to examine the intervening geological medium. Single-hole reflection survey was also carried out to scan the medium around the borehole. The result of cross-hole GPR survey is a plot of radar wave attenuation along the horizontal plane of survey. The attenuation plot clearly delineated the zone affected by past mining activity; however cavity type features were not mapped. Even though exploratory drill holes showed evidences of old workings up to 250-260 m depth from the ground surface, the present borehole GPR investigation from an underground gallery at 298 m depth confirmed that old workings did not cross the horizontal plane of investigation.
Seismic refraction has long been adapted for various types site characterization studies. The method has evolved from the classical theory based on Huygens's principle involving straight propagation paths representing spherical wave front heading direction into many novel considerations involving curved-ray-path and cell based finite element techniques for tracing the propagation of seismic waves in the subsurface media and deriving the velocity of the body waves as closely approximated to reality as possible, within the theoretical and computational limits of the routines. In view of multifarious options available in the market for plotting and visualizing and interpretation of seismic sections it becomes important to understand the influence of manual and automated processing techniques on reasonability, accuracy and reliability of velocity sections derived from such techniques. This paper comparatively evaluates the merit and limitation of each of the techniques w.r.t to the site conditions, the depth of investigation and resolution. This study emphasizes the need of optimal selection of field survey and processing parameters with due regard to the constraints experienced during acquisition and processing
Geophysical investigations prior to constructions stage are often met with difficult site conditions particularly over shallow water bodies. Due to site constraints, standard data acquisition procedures, either onshore or offshore may not be suitable in such cases. In this situation designing of site specific investigation technique becomes necessary to explore the subsurface condition. In one such case study for locating the barrage axis with rockmass in the foundation, seismic refraction survey was deployed. The survey data was acquired both onland and onwater-body to map the bed rock depth and geological features in the study area. A combined section was then obtained by joining lines on surface and over waterbody.Based on data processing, the subsurface layers wereclassified as compact soil, weathered rockmass layer with boulders, jointed rockmass layer and hard rock layer based on the seismic velocity range. These results were correlated with the available data on geological situation, Standard Penetration Test (SPT) values and laboratory studies. Interpretation of survey data hinted at the shifting of river course away from the left bank by at least by 50-60m. Considering the site conditions, it was suggested that the left bank abutment may be extended by 30m for coupling with the in-situ hard rock.
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